As a structure of a conventional spiral-type separation membrane element, those are known in which a single laminate or plural laminates of a separation membrane, a feed-side flow passageway member, and a permeate-side flow passageway member are wound around a porous hollow water-collecting tube (See, for example, the patent document 1).
In this membrane element, a feed solution is supplied from one end surface and is filtered by the separation membrane while flowing along the feed-side flow passageway member, and the concentrated liquid is taken out from the other end surface. The transmitted liquid that has been filtered by the separation membrane flows along the transmittance-side flow passageway member, flows in through the holes of the water-collecting tube, and flows within the water-collecting tube. For this reason, in order to enhance the energy efficiency of the separation operation, the smaller the pressure loss generated when the feed solution flows along the feed-side flow passageway member is, the more preferable it is.
As the feed-side flow passageway member, those having a net shape are typically used, and have a function of restraining the concentration polarization by promoting the surface renewal of the membrane surface while at the same time ensuring the flow passageway on the element feed-side. In order to restrain the concentration polarization, there is a method of increasing the linear speed of the membrane surface by reducing the thickness of the passageway member. However, this has a problem such that the floating components in the supplied liquid clog the flow passageway, or a problem such that the needed dynamic power of the pump that transports the supplied liquid will be large.
Thus, in order to restrain the concentration polarization caused by the feed-side flow passageway member and to reduce the pressure loss of the feed-side flow passageway, a feed-side flow passageway member is known in which the thread constituting the net is tilted relative to the flow direction of the feed solution and the interval between the intersections in the flow direction of the feed solution is made larger than the interval between the intersections in the direction perpendicular to this (See, for example, the patent document 2).
Also, a feed-side flow passageway member is known in which the diameter of the lateral thread that intersects relative to the flow direction of the feed solution is made smaller than the diameter of the longitudinal thread that is parallel to the flow direction (See, for example, the patent document 3).
However, in any of the above-described feed-side flow passageway members, the cross-sectional shape of the net-constituting thread is circular, and also the thickness is constant, so that there has been a limit in attaining compatibility between the restraint of the concentration polarization caused by the feed-side flow passageway member and the reduction of the pressure loss of the feed-side flow passageway.
PATENT DOCUMENT 1: Japanese Patent Application Laid-Open (JP-A) No. 10-137558
PATENT DOCUMENT 2: JP-A No. 2000-437
PATENT DOCUMENT 3: JP-A No. 2004-283708